Power conditioning circuits have long been used to protect sensitive load equipment from transients caused by lightning strikes, noise, and other power line disturbances. Traditionally, filter elements are used in the line and neutral conductors which trap and/or shunt unwanted power frequencies away from the load. See, for example, Speet et al. U.S. Pat. No. 4,814,941 and Taylor et al. U.S. Pat. No. 5,490,030.
Muelleman U.S. Pat. No. 5,448,443 discloses a power conditioning device and method including an isolation transformer having primary and secondary sides and a ground impedance connected between the secondary side of the isolation transformer at a safety ground and an earth ground. The Muelleman device prevents ground current loops by redirecting transient ground currents to neutral, but does not provide current limiting or noise suppression.
Redburn et al. U.S. Pat. No. 6,166,458, owned by the assignee of the present application and the disclosure of which is hereby incorporated by reference herein, discloses a power conditioning circuit for conditioning single-phase power supplied by a power source at a nominal frequency over line, neutral, and ground conductors. The power conditioning circuit includes first through third impedances coupled to the line, neutral, and ground conductors, respectively, and to output lines wherein each of the impedances prevents power at frequencies greater than the nominal frequency from reaching the output lines. Preferably, the first through third impedances comprise first through third inductors coupled in series with the line, neutral, and ground conductors, respectively, between the power source and the output lines. Means may also be provided for limiting voltages across the line, neutral and ground conductors. The limiting means may comprise at least one metal oxide varistor or at least one zener diode. The power conditioning circuit is simple in design, yet effective to limit damaging transients.